Porous ceramic preforms typically are prepared using powder pressing techniques and find application as catalysts, filters or as preforms in the manufacture of metal/ceramic composites by molten metal infiltration techniques.
In U.S. Pat. Nos. 5,114,886 and 4,777,152, there are described the most commonly used methods for the production of porous ceramic parts.
The '886 patent teaches a method which involves mixing a coarse ceramic powder having spherical particles with a finer powder, compacting the admixed powders and a binder to form a green body and sintering the body until the powders fuse together at their contact points. The porosity is created by the spaces left between the particles. A first major disadvantage of this method is that the strength of the porous preform is low because the linkages between the powder aggregates are limited. Secondly, there is only limited and indirect control over the amount, size and shape of the porosity. The amount of porosity is determined primarily by the size of the selected ceramic powders. The size and shape are determined by the arrangement and size of the ceramic aggregates. High levels of porosity lead to extreme fragility of the preform.
K. Tsukada in U.S. Pat. No. 4,777,152 discloses a porous preform prepared from two crystalline forms of silicon carbide. In this case, the ceramic aggregates are plate-like in shape, which improves the linkages between aggregates, thereby improving the preform strength. Again, the process provides indirect control over the porosity and can only be used to make porous silicon carbide ceramics.
A variant is exemplified by Canadian Laid-Open Application 2,121,864. Silicon carbide powder is admixed with a preceramic organopolysiloxane prior to pressing and sintering. During sintering, the organopolysiloxane reacts to form silicon carbide and carbon which not only assists in binding the original silicon carbide powder together but also creates porosity. As in the '152 process described supra, control over porosity is limited and the process is restricted to the fabrication of silicon carbide preforms.
Additionally, the disadvantages inherent in such ceramic preforms are that they lack accuracy in shape and in dimensional control.
Ceramic preforms having graded porosity are very difficult to produce by the above-described processes.
U.S. Pat. Nos. 5,019,539, 5,164,347, 5,015,610 and 5,139,977 disclose processes for the production of porous and dense ceramic composites. The basic process involves the oxidation of a molten metal which is directed into a ceramic powder bed positioned thereabove. Porosity is created by controlling the oxidation conditions. Deleteriously, it is not possible to obtain direct control of the porosity. However, as exemplified in U.S. Pat. No. 5,019,539, it is possible to obtain preforms having some degree of gradation in their porosity. This is attained by changing the particle size of the ceramic powder at differing locations in the ceramic bed. However, the graded porosity cannot be controlled either in a discrete, stepwise fashion or controlled over a wide range.
Composite products consisting of a metal matrix and reinforcing phase, such as ceramic particles, show enhanced material properties in combining some of the stiffness and wear resistance of the reinforcing phase, with the ductility and toughness of the metal matrix. However, the high temperature mechanical properties, wear resistance and corrosion resistance of the metal phase can be a limiting factor in certain applications of these composites. Thus, it has been determined that by substituting the metal matrix with an intermetallic matrix, such as nickel aluminide, a composite exhibiting much improved properties is formed.
Illustrative of the prior art with respect to the preparation of metallic/ceramic composites using infiltration techniques are the disclosures of U.S. Pat. No. 4,033,400 issued to Gurwell et al. The patent provides a biskeletal composite which is formed of a bonded silicon nitride host. A metallic infiltrant material is heated to its liquidus temperature and forced by pressure into the host. Unfortunately, in order to apply the requisite high pressure complex apparatus is needed. Additionally, the preform is open to damage by the applied pressure. Furthermore, the porous preform is fabricated by sintering ceramic powder, thus giving rise to the known attendant disadvantages of sintering processes.
A variation of the process for manufacturing metallic/ceramic composites is disclosed in U.S. Pat. No. 4,828,008 which teaches the selection of an infiltrating alloy and gaseous atmosphere functional to cause the metal to spontaneously infiltrate a loose ceramic powder bed. This is a result of the excellent `wetting` properties existing between the ceramic and the metal alloy. The process is limitative in that aluminum alloys must be used, said alloys must contain at least 1 wt % magnesium and must be carried out in a nitrogen atmosphere. An inherent disadvantage of the process resides in the fact that it can only produce composites wherein the ceramic is a dispersed particulate phase having a metal matrix.
U.S. Pat. No. 5,372,777 describes a process for producing a graded, composite microstructure. The process involves complicated treatments including settling of the particles in conjunction with repeated infiltration steps whereby direct control over the degree of grading is difficult.
Shaped intermetallic/ceramic/metal components may be formed utilizing thermomechanical forming routes such as extrusion or forming. Alternatively, casting processes may be used. Both methods are expensive and involve complex techniques.
Another, more commonly employed approach for the manufacture of components formed of intermetallic/ceramic/metal composite materials is that of powder metallurgy. Exemplary processes are described in U.S. Pat. No. 4,919,718, or the paper by Misiolek and German in "Materials Science and Engineering", Volume A 144 (1991) pp 1-10. An alternative process founded on powder metallurgy is given by McCoy and Shaw in "Advances in Powder Metallurgy and Particulate Materials" (1994) Volume 5. A major drawback with powder metallurgical based processes resides in the difficulty in obtaining fully dense parts. Furthermore, it is virtually impossible to produce parts having graded microstructures and to eliminate the dispersion of the ceramic reinforcement phase within the intermetallic metal matrix.
Processes for the fabrication of ceramic components per se are various and well-documented in the literature. Amongst such processes is the tape casting process which is primarily known for the manufacture of ceramics used in electronic applications as described by Mistler, R. E. et al. (1978) Tape Casting of Ceramics, in Ceramic Processing Before Firing G. Y. Onoda and L. L. Hench, eds., Wiley-Interscience, 411-448.
Tape casting techniques involve, in general, preparing a colloidal suspension comprising a ceramic powder, a binder system, a plasticizer and a solvent. The suspension is cast into a thin sheet, and air dried yielding a green body. The tape is subjected to a burnout-cycle to remove pyrolysable slurry additives forming a friable brown body which is subsequently sintered to yield the final product.